Fetal Blood Velocity Waveforms in Normal Pregnancies: A Longitudinal Study

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Abstract
Blood velocity in the umbilical artery, fetal descending aorta and fetal middle cerebral artery was recorded by means of 3.5 MHz pulsed Doppler ultrasound in 22 normal pregnancies. The measurements were performed longitudinally between 25 and 42 weeks of gestation. The maximum blood velocity curves were analysed for pulsatility index (PI), systolic/diastolic velocity ratio (S/D ratio) and in the middle cerebral artery also for the resistance index according to Pourcelot (RI), In the umbilical artery, all variables decreased significantly with increasing gestational age as an expression of the decreasing placental vascular resistance. PI in the descending aorta remained fairly constant during the examined period of gestation. In the middle cerebral artery, all three variables (PI, S/D ratio, RI) decreased significantly as gestation advanced. With the exception of the umbilical artery at 28-32 weeks, PI and S/D ratio were not influenced by the heart rate in any of the vessels examined. Diagrams of the gestational age-related reference values (means .+-. 2 SD) for the variables of the blood velocity waveforms were established. Additionally, a ratio of the PI and S/D was calculated for the umbilical artery and middle cerebral artery.